In cost-sensitive mobile transmitters, performance trade-offs must be carefully managed to achieve high efficiency and high output power at the required gain and linearity. With an intrinsically nonlinear power amplifier (PA) itself, the only way to achieve a better linear operation is to restrict the dynamic range of signals to a small fraction of the PA's overall capability. Unfortunately, such a restriction in the dynamic range to achieve a more linear operation is quite inefficient since it requires the construction of an amplifier that is much larger in size and consumes more power.
With the demand to increase data transmission rates and communication capacity, Enhanced Data rate for GSM Evolution (EDGE) has been introduced within the existing GSM (Global System for Mobile communications) specifications and infrastructure. GSM is based on a constant envelope modulation scheme of Gaussian Minimum Shift Keying (GMSK), while EDGE is based on an envelope-varying modulation scheme of 3π/8-shifted 8-phase shift keying (8-PSK) principally to improve spectral efficiency. Because of this envelope-varying modulation scheme, EDGE transmitters are more sensitive to PA nonlinearities, which can significantly and negatively affect the performance of an EDGE handset. As a result, EGDE transmitters require accurate amplitude and phase control with additional blocks to compensate for distortion caused by the PA nonlinear characteristics and non-constant envelope variation.
To provide for efficiently amplified signal transmissions, many polar transmitter architectures have been proposed in the form of either an open-loop with digital predistortion scheme or a closed-loop with analog feedback scheme. First, in the conventional open-loop with digital predistortion scheme, the PA is characterized by calibration data including power, temperature, and frequency. The calibration data is then stored in look-up tables. The correct coefficients for the operating conditions from the look-up table are selected by digital logic and applied for predistortion. The DSP-based linearization can provide an accurate, stable operation as well as easy modification by the power of software programming. However, this technique requires time-consuming calibration on the production line to compensate for part-to-part variations and cannot easily correct any aging effect in the system. When employing a path for reflecting changes at the PA output to linearization, the circuitry becomes large and costly and consumes a considerable amount of DC power.
Second, a polar loop envelope feedback control is generally used for analog linearization. In such a feedback control structure, a precise receiver has to be included within the transmitter and the control loop bandwidth should greatly exceed the signal bandwidth. In addition, the intrinsic gain reduction characteristic in the negative feedback may cause a severe restriction to amplifiers that do not have enough transmission gain. Additionally, conventional polar loop systems feed back both distortion and signal power, thereby reducing the stability of the polar loop systems. Likewise, power amplifiers used in these conventional polar modulation systems are operated at highly nonlinear switching modes for efficiency so the cancellation of high-order distortion components becomes more important.